Island of stability
An island of stability is a region in the (Z,N) (Z,A) plane in which nuclei are predicted to have long half-lives against decay by spontaneous fission. It does not imply actual long life because decay by other modes, particularly alpha decay, may be quite rapid. Protons and neutrons must each occupy a unique state within a nucleus, in the same way that each electron occupies a unique state within an atom. These states form shells and subshells analogous to those formed by electron states. A nucleus, unlike an atom, has two separate sets of quantum states, one for protons and one for neutrons. One big difference between nuclei and atoms is that electron wavefunctions have spherical symmetry because the nucleus can be treated as a tiny, spherical charge center. Most nuclei are not spherical, so proton and neutron wavefunctions are more complicated. Some predictions exist, but only for nuclei close to those which have been observed. Occupancy of a nucleus' proton and neutron shells affects stability of the nucleus, with a maximum reached when all shells are fully occupied. (Just as atoms are most stable (most inert) when all their electron shells are fully occupied.) The term "shell closure" is also used to refer to full occupancy for a nucleus' proton or neutron shells. The terms "proton magic" and "neutron magic" are also used to refer to the same thing, and the term "doubly magic" is used when both sets of shells are fully occupied.. Occupancy of a nucleus' neutron and proton shells determine stability of the nucleus in the same way that electron shell occupancy determines an atom's chemical reactivity. Maximum stability is reached in magic nuclei, just as atoms are most inert when they have filled electron shells. Neutron shell closure appears to have a much stronger effect on nuclear stabililty than proton magicity(1). Islands of stability occur in the vicinity of neutron magic numbers, and particularly where one or two elements in the region are doubly-magic. Shell closure does affect stability against alpha decay, but has a much stronger effect on fission (and probably cluster decay). As a result, nuclei in an island of stability tend to decay by alpha or beta emission instead of fission. Doubly-magic nuclei can be used to identify where islands of stability are likely to occur. Neutron shell closures have been predicted at N = 184, 228, 308, 406, 524, 644, and 772; with N = 196 and 318 possible additional closures. There is more debate about the location of proton shell closures, but Z = 126, 138, 154,164, 210, and 274 have been identified as likely to be proton magic. Islands of stability are likely to occur near nuclides (Z,A) = (126, 310), (126, 354), (126, 434), (138, 322), (138, 366), (138, 446), (154, 382), (154, 462), (164, 472), (164, 570), and (210, 734). Additonal islands are also probable. By far the best-studied such region is around N = 184. Nuclei in this region form what are called "superheavy elements". Although nuclei between Z = 110 and Z = 118 which have been observed are short-lived, most decay by alpha emission. In nuclei of this size, that is experimental evidence that an island of stability does exist near Z = 184. 1.) "Magic Numbers of Ultraheavy Nuclei; V.Y Denisov; Physics of Atomic Nuclei, vol 68, no. 7, 2005, pp 1133-1137; www.researchgate.net / publications / 225734594